Contraction Bands: Differences between Physiologically vs. Maximally Activated Single Heart Muscle Cells
High resolution interference and phase microscopy were used to inspect the striations’ appearance in shortening rat heart cells. Isolated cells were treated with detergent so that shortening could be graded by addition of calcium. Upon activation sarcomeres shortened to form (a) contraction densities in the middle of the A band at 1.7 um, (b) disappearance of the I bands and (c) phase brightening of the A bands at 1.8 um, and (d) dense Cz contraction bands at shorter lengths. These changes are totally consistent with the uniform sliding of myofilaments of previously accepted fixed dimensions. However, the striated patterns differed significantly in intact cells which were electrically stimulated to shorten. Here individual A bands remained distinct, without phase brightening or contraction band formation despite sarcomere shortening to less than the length of the A band as measured in the unstimulated cell. Maximal activation of intact cells by barium contracture elicited the full sequence of striation changes (a-d) seen in the chemically skinned cells. Light diffraction analysis gave comparable interpretation, i.e., the protein within the shortened sarcomnere in the physiologically activated cardiac cell is more narrowly distributed than expected for thick filaments of fixed dimensions. These optical differences may reflect the restricted presence of the globular myosin heads at the ends of the cardiac sarcomere. This situation would explain the narrow range of the cardiac length-tension relation.
KeywordsIntact Cell Thin Filament Sarcomere Length Thick Filament Cardiac Muscle Cell
Unable to display preview. Download preview PDF.
- Anversa, P., Loud, A.V., Giacomelli, F. and Wiener, J. (1978). Absolute morphometric study of myocardial hypertrophy in experimental hypertension. II. Ultrastructure of myocytes and interstitium. Lab. Invest. 38: 597–609.Google Scholar
- Brown, L., Gonzalez-Serrates, H. and Huxley, A.F. (1970). Electron microscopy of frog muscle fibre in extreme passive shortening. J. Physiol. 208: 86–88 P.Google Scholar
- Fabiato, A. and Fabiato, F. (1976). Dependence of calcium release, tension generation and resting forces on sarcomere length in skinned cardiac cells. Eur. J. Cardiol. 4/suppl. 1327.Google Scholar
- Fujime, S. (1975). Optical diffraction study of muscle fibres. Biochim. Biophys. Acta 3799: 227–238.Google Scholar
- Dewey, M.M., Levine, R.J.C., Colflesh, D., Walcott, B., Brann, L., Baldwin, A. and Brink, P. (1979). Structural changes in thicck filaments during sarcomere shortening in Limulus striated muscle. In: Cross-Bridge Mechanism in. Muscle Contraction, 3–19, eds. Sugi, H. and Pollack, G.H., Baltimore: Univ. Park Press.Google Scholar
- Hasselbach, W., Somer, J.R. and v.Graff, H. (1975). A-band shortening in contracted skeletal muscle fibrils. Fed. Proc. 34: 474.Google Scholar
- Herman, L. and Dreizen, P. (1971). Electron microscopic studies of skeletal and cardiac muscle of a benthic fish. I. Myofibrillar structure in resting and contracted muscle. Am. Zool. 11: 543–557.Google Scholar
- Hill, L. (1977). A-band length, striation spacing and tension change on stretch of active muscle. J. Physiol. 226: 677–685.Google Scholar
- Page, S.G. (1974). Measurement of structural parameter in cardiac muscle. CIBA Foundation Symposium 24: 11–26, Elsevier, Amsterdam.Google Scholar